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Understand WDM in One Minute

WDM, Wavelength Division Multiplexing. It is the technology which combines the lights with different wavelength then goes through one fiber, the lights are separated again once it reaches another side, each light goes to their own destination.

WDM, Wavelength Division Multiplexing

To make the WDM works well and efficiently, the wavelengths must be allocated properly and there must be an interval between each wavelength.  Otherwise, they will interfere with each other.

As we knew, the speed of light = wavelength x frequency. The speed of light is a fixed value, so Different wavelength means different frequency. Wavelength interval reflects the frequency interval.

The too big interval will limit the transmission capacity while too small interval will make signal interference.


In the beginning, the interval is 10nm or more due to the technology limit.  That is CWDM,  Coarse WDM. Normally, the interval is 20nm. Here are the wavelengths for CWDM.

NO Wavelength(nm)
1 1471
2 1491
3 1511
4 1531
5 1551
6 1571
7 1591
8 1611
9 1271
10 1291
11 1311
12 1331
13 1351
14 1371
15 1391
16 1411
17 1431
18 1451

According to ITU. The CWDM wavelength range is from 1271 to 1611.

But 1270-1470nm have an apparent attenuation. So 1471 to 1611nm with the high priority to be chosen.

As technology develops, the interval can be smaller and smaller. Less than 10nm, it is called DWDM, Dense WDM.

The wavelength interval of DWDM can be 1.6nm, 0.8nm, 0.4nm, 0.2nm, it supports 40 channels (wavelengths), 80 channels or 160 channels, maximum 192 channels.  The wavelength range is 1525~1565nm(C band) and 1570nm ~ 1610nm(L band)

Here are the comparisons between CWDM and DWDM.

Name Coarse Wavelength Division Multiplexing Dense Wavelength Division Multiplexing
Wavelength Interval 20nm 0.8nm, 0.4nm, 0.2nm, 1.6nm
Wavelength Range 1270~1610nm 1525~1565nm


Channels 18 40, 80,160, 192
Cost Low High
Transmission distance short Long
Modulation electrical temperature

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